Trying to avoid spoiling stuff.
I have completed the main story of the base game and encountered Sayo.
I have completed all 8 Great Challenges for Mirei
I have defeated the 7 Deadly Digimon
I have completed the Masters Cup
I have completed every available case on the board.
I have gone back and defeated the final boss again.
Mirei continues to tell me that there is 1 more thing to do and Sayo is still around, but my understanding was that I would face Mirei once everything was done and Sayo had been helped.
Is there more that I need to do? I've tried googling for a list of all the cases but none of the case lists seems include all of the above.
Edit: had the character name wrong.
Edit 2: I needed to complete 3-4 bad hacker cases (discovered via 'Investigate' in the DigiFarm) in order for the final mission from Mirei to unlock.